1. Field of the Invention
The present invention relates to a motor stator wherein teeth radially project from a plurality of positions spaced in the peripheral direction of an annular stator core, and wherein a winding is wave-wound around the teeth.
2. Description of the Related Art
In a motor having a stator in which windings having a plurality of phases are wound around slots formed between teeth, short pitch winding and distributed winding are used to suppress the generation of torque ripple or abnormal torque harmonics due to the slots and to obtain smooth rotation. See Japanese Patent Application Laid-open No. 4-721461 and Japanese Patent No. 2765764.
However, when short pitch winding or distributed winding is used, as shown in FIG. 12A, in a winding 04 wound around slots 03 between teeth 02 of a stator 01, bridging portions 04a, which do not contribute to the generation of torque, become long, leading to problems. For example, the winding resistance increases, which results in a deterioration of efficiency. Also, when opposite ends of the bridging portion 04a are bent through 90°, an axial projection height H of the stator 01 increases, which makes it very difficult to reduce dimensions.
When wave winding, as shown in FIG. 12B, is used, the length of the bridging portions 04a of the winding 04 is shortened, wherein the winding resistance is decreased and there is an enhancement or improvement in efficiency. Also, the axial projection height H of the bridging portions 04a is reduced, which results in a reduction of dimensions. However, since torque ripple or abnormal torque harmonics, due to the slots 03, occurs by wave-winding the winding 04, the slots 03 must be inclined or skewed, as shown in FIG. 12C.
That is, as shown in FIG. 11, a force F that an unskewed length L of a conductor receives is represented by:F=I·B·L·sin ωtwhere I·sin ωt denotes a current and B denotes a magnetic flux density.
On the other hand, a force dF that an infinitesimal portion dL of the conductor having a skew angle θ receives is represented by:dF=I·B·sin {ωt+(cos θ/L)·x}·cos θ.Consequently, a force Fsk received by the entire conductor is represented by:
                                                        Fsk              =                            ⁢                                                ∫                                      -                                          L                                              2                        ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                                                                                  L                                          2                      ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                      ⁢                                                      I                    ·                    B                    ·                                          sin                      ⁡                                              (                                                                              ω                            ⁢                                                                                                                  ⁢                            t                                                    +                                                                                                                                                      cos                                  ⁢                                                                                                                                          ⁢                                  θ                                                                ⁢                                                                                                                                                              L                                                        ⁢                            x                                                                          )                                                              ·                    cos                                    ⁢                                                                          ⁢                  θ                  ⁢                                      ⅆ                    x                                                                                                                          =                            ⁢                                                ∫                                                            ω                      ⁢                                                                                          ⁢                      t                                        -                                          γ                      2                                                                                                  ω                      ⁢                                                                                          ⁢                      t                                        +                                          γ                      2                                                                      ⁢                                                      I                    ·                    B                    ·                    sin                                    ⁢                                                                          ⁢                                                            x                      ~                                        ·                                                                                            cos                          ⁢                                                                                                          ⁢                          θ                                                ⁢                                                                                                                      L                                        ·                    cos                                    ⁢                                                                          ⁢                  θ                  ⁢                                      ⅆ                                          x                      ~                                                                                                                                                              x                ~                            =                            ⁢                                                ω                  ⁢                                                                          ⁢                  t                                +                                                                            cos                      ⁢                                                                                          ⁢                      θ                                        L                                    ⁢                  x                                                                                                        =                            ⁢                              I                ·                B                ·                                                                            L                      γ                                        ⁡                                          [                                                                        -                          cos                                                ⁢                                                                                                  ⁢                                                  x                          ~                                                                    ]                                                                                                  ω                      ⁢                                                                                          ⁢                      t                                        -                                          γ                      2                                                                                                  ω                      ⁢                                                                                          ⁢                      t                                        +                                          γ                      2                                                                                                                                              =                            ⁢                                                I                  ·                  B                  ·                                      L                    γ                                    ·                  2                                ⁢                sin                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  t                  ·                  sin                                ⁢                                                                  ⁢                                  γ                  2                                                                                        (                  Equation          ⁢                                          ⁢          1                )            and a skew factor ksk=Fsk/F is represented by:
                    ksk        =                              Fsk            F                    ⁢                                          ⁢                                          =                                                                      I                  ·                  B                  ·                                      L                    γ                                    ·                  2                                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  t                  ·                  sin                                ⁢                                  γ                  2                                                                              I                  ·                  B                  ·                  L                  ·                  sin                                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                t                                      ⁢                                                  ⁢                                                  =                                          sin                ⁢                                  γ                  2                                                            γ                2                                                                        (                  Equation          ⁢                                          ⁢          2                )            Furthermore, a skew factor ksq for harmonics of order q is represented by:
                    ksq        =                              sin            ⁢                                                  ⁢            q            ⁢                          γ              2                                            q            ⁢                          γ              2                                                          (                  Equation          ⁢                                          ⁢          3                )            
As is clear from Equation 2 and Equation 3, the greater the skew angle θ (the greater the γ), the smaller the skew factors ksk and ksq. In other words, as in the case where the above-mentioned short pitch winding or distributed winding is used, torque fluctuation of the motor may be suppressed concurrently with the reduction of motor torque.
As shown in FIG. 12C, although torque fluctuation of the motor is suppressable by skewing the wave-wound winding 04, the length of the bridging portions 04a cannot be shortened compared with the unskewed winding 04 shown in FIG. 12B. Instead, the winding 04 bends sharply in certain regions a to increase the projection height H of the bridging portions 04a, resulting in the axial dimensions of the stator 01 increasing.